question

What kind of Grid is available on site ... maybe comparable with our TN-C-S network in Germany ...
It sounds almost as if there are several phases and a neutral / combined neutral / protective earth conductor on site.
If this is the case, the loss of the neutral/protected earth conductor will cause a destructive displacement of the neutral point ... and thus to the destruction of many connected devices by increased mains voltage.
I think a normal device installation with Multi Plus leads also here to the damage of the device ... since more than the permitted mains voltage is present for a short time. An additional overvoltage shutdown should be insatlliert here in any case BEFORE the Multi ... so that this switches off as quickly as possible and puts the Multi in the inverter mode ...

A "floating neutral" is probably the most destructive condition likely to be seen in most systems. As netrange says, a neutral failure will shift the voltage from one phase to other phases resulting in a large over voltage to appliances.

The best protection against a floating neutral is to not use it !! In stead, use an isolation transformer between the hot legs and create new split or 3-phase power with a "local neutral".

The next best protection would be a fast-acting relay to disconnect all phases of the grid when the voltage of any one phase exceeds a preset value. The trouble is, a neutral that suddenly disconnects can produce high voltage on one leg instantly. So the relay may not open soon enough to prevent damage.

A Victron Multi or Quatro will (eventually) detect an over-voltage condition on it's input and open the AC input relay. Like the system above, that may not be fast enough to prevent inverter/charger core damage or damage to connected loads.

There is no hard specification of Victron inverters to indicate if damage would occur should the voltage at it's input suddenly double. Some posters on this forum have indicated a double voltage condition will damage the input circuits of a Multi or Quatro. This damage can occur even if the AC input relay is open at the time of the fault.

Incorrect grid voltage is also a risk especially for mobile power systems that may see 120, 120/240 or 230/460 volt grid power sources. Fortunately, these voltage changes are not likely to occur after initial connection where the appropriate step up/down transformers can be switched into the grid connection before any devices.

Thank you guys for your answers, thats very interesting! I have also been told in the meantime that what we have experienced is pretty much the worst case scenario ...

@Netrange I cannot say for sure, but I think its an TT net. And yes, we're already planning an overvoltage protection, which of course will be installed before the Victron equipment.

Our current consumption is currently only about 20-40 kW per day, but two more buildings are planned which will have more technical equipment. We are currently in the process of coordinating this, but three multi/quattro units with 8 or 10 kVA are currently being discussed. A isolation transformer would then have to be dimensioned accordingly, right?

@Kevin Windrem Thanks for your hint to the isolation transformer. I think a combination of overvoltage protection and an isolation transformer will be the way to go.

By the way, just before that we had a lightning strike on site, which also cost a couple of electrical appliances. So that topic comes along as well. I can highly recommend volunteering for an NGO, it never gets boring!